The invention concerns a photodiode array in accordance with the precharacterizing part of claim 1. Such a photodiode array is used, for example, for measuring the absorption spectrum of a sample substance to derive information concerning the chemical composition of the sample and the amounts of the individual constituents in the sample.
Such a photodiode array is known in the art from European patent EP 0 519 105 B1. This conventional photodiode array can be used in a liquid chromatograph for analyzing the substances eluting from the chromatographic column. It comprises a light source emitting a broad spectrum of ultraviolet and visible radiation and an optical system for focussing the beam onto a sample cell through which the sample substances to be analysed flow. Depending on the specific substances flowing through the cell, the sample absorbs certain characteristic spectral portions of the radiation entering the sample cell so that the spectral composition of the radiation leaving the cell is indicative of the sample substances.
In this spectrometer, the spectrum of the radiation leaving the sample cell is extracted using a diffraction grating disposed in the optical path behind the cell. The diffraction grating directs light rays of different wavelengths into different directions. A linear photodiode array is disposed to receive the light diffracted by the grating. Each diode therefore receives light corresponding to a different wavelength range. The electrical signals produced by the light impinging on each photodiode are read out by a readout circuit and converted to digital data values corresponding to the intensity of the light impinging on the specific diode. These data values are then displayed as a function of wavelength in any convenient form, for example on a CRT screen.
The semiconductor based photodiode array comprises a plurality of photosensitive elements which are connected via electronic switches to a common output line, e.g. a video line which, in turn, is connected to a charge amplifier. Each photosensitive element has an associated capacitor representing the junction capacitance of the photodiodes. The combination of a photosensitive element and associated capacitor is also called a "photocell".
Light impinging on the photosensitive material generates charge carriers to discharge these capacitors. The photocell capacitors are initially charged to a predetermined value and are discharged by the photocurrent generated by the photocells when light impinges thereon. The amount of charge which is necessary to recharge the capacitors to their original values causes a voltage change at the output of the charge amplifier proportional to the light intensity on the photodiode.
A photodiode array comprises a plurality of photocells each generating output signals. Normally, the photodiode array operates in an integrating mode, whereby the photocells are processed sequentially. This is, however, associated with the problem of so called spectral distortion. Particularly in spectrophotometers used to detect the sample substances eluting from the column of a liquid chromatograph, the sample to be analyzed changes as a function of time. Since the signals from the individual photocells are processed sequentially the signals corresponding with certain wavelengths can become time distributed. Another problem is that a single A/D converter is normally used to sequentially convert the signal from individual photodiodes of the photodiode array. Since the number of photodiodes is usually very large, i.e. 1024 photodiodes, the conversion rate of the A/D converter has to be very high, e.g. above 100 kHz, to ensure high measuring accuracy. Such A/D-converters are rather complex and expensive.
For this reason, a parallel architecture in accordance with EP 0 519 105 is preferred for the photodiode array. The signals of each channel, with its own converter, are each generated simultaneously. The requirements for the A/D converters of each channel can therefore be reduced and the measuring accuracy of a time changing sample concentration can be improved.
In accordance with an improved integration of the photodiode array on e.g. a silicon chip, a charge balance type of photodiode array is preferentially used. With this type of photodiode array, the accumulated charge is removed in defined charge packets from a switchable dumping-capacitor. The frequency of charge dumps required to keep the system in balance is proportional to the photocurrent generated by the individual photodiode. For effecting the A/D conversion, each photodiode is connected to the summing node of an integrator which continuously accumulates the charge corresponding to the photocurrent. The output signal of the integrator is periodically compared to a predetermined signal level, i.e. by a suitable comparator, and in response to these comparisons, charge dumps to and/or from the integrator are performed to keep the output signal at a predetermined level. The number of such dumps during a predetermined time interval are counted, i.e. by a logical counter. The counted number is a digital signal representing the actual photocurrent.
In a preferred embodiment of this conventional photodiode array, a current mirror, i.e. a "Wilson current mirror", is used to amplify and to reverse the photocurrent. This embodiment is useful since the photocurrent varies in dependence on differing applications and light intensities. The current mirror is inserted in the photocurrent path between the photocell and the summing node of the integrator circuit, to decouple the junction capacitance of the photocell from the input signal of the integrator at the summing node.
The above mentioned photodiode arrays are used in many different kinds of spectrophotometers for a plurality of different applications. In some cases the light source is a flash light type. This type of light source provides higher light intensities. Their durability and their efficiency is much better than that of continuous light sources.
Disadvantageously, the charge balance photodiode arrays can not operate in conjunction with such flash type light sources, since a charge balance type photodiode array immediately saturates when the photocurrent exceeds a defined limit given by the size of the dumping capacitor and its associated maximum dump rate.